1. Field of the Invention
This invention relates to the treatment of gold ores of sedimentary origin which are characterized by a significant content of organic carbonaceous and sulfidic mineral materials.
2. The Prior Art
Sedimentary gold-bearing ores containing indigenous organic carbonaceous material are notoriously refractory to standard cyanidation treatment for the recovery of their gold content. Investigation into the cause of this problem has indicated that the carbonaceous material comprises active carbon and long-chain organic compounds. The active carbon appears to absorb the gold cyanide complex [Au(CN).sub.2 .sup.-] from cyanide leaching solutions, and the long-chain organic compounds appear to form stable complexes with the gold. In order to overcome this sequestering of the gold, experiments have been conducted by the United States Bureau of Mines in which they used a wide variety of oxidation pretreatment systems including ozone, sodium hypochlorite, calcium hypochlorite, permanganates, perchlorates, chlorates and oxygen. Of these the hypochlorites appeared to be the most effective, and although an elevated temperature was required it was found that temperatures above about 60.degree. caused premature decomposition of the hypochlorites. Consequently, it was determined that a temperature within the range of 50.degree. to 60.degree. C. (122.degree. to 140.degree. F. was required for effective hypochlorite treatment to render the gold component of the ore amenable to standard cyanidation treatment. At room temperature (about 70.degree. F.), the hypochlorite treatment requires several days for the required reaction to make the gold content of the ore available for recovery by cyanidation.
In a previous patent issued to Wilbur J. Guay, U.S. Pat. No. 3,846,124 of Nov. 5, 1974, it was shown that the recoverability, by standard cyanidation, of the gold content of organic carbon-containing sedimentary gold bearing ores is increased by slurrying the ground ore with water, charging chlorine gas to the slurry to the extent that the slurry will absord the chlorine, and holding the thus treated slurry for at least 6 hours while maintaining it at the chlorination treatment temperature of about 70.degree.-85.degree. F.
It has now been determined that a substantial portion of the oxidation of elemental carbon and organic carbonacous compounds can be accomplished with oxygen, or air by bubbling either of these reagents through a ground slurry of ore and water for periods of 8 to 24 hours in a temperature range of 167.degree. to 212.degree. F. It has also been determined that pyrite, FeS.sub.2, is oxidized by the aeration treatment, and in the case of gold-bearing pyrites additional gold is thus released for later recovery by cyanidation. Other gold-bearing sulfides would also be oxidized by the aeration treatment, thus releasing gold for later recovery by cyanidation. Inasmuch as the oxygen or air treatment does not oxidize all of the organic compounds present it is necessary to follow the oxygen or air treatment by bubbling chlorine through the ground slurry of ore and water to oxidize the more difficulty oxidizable compounds for an additional period at a rate such that the chlorine will be substantially completely adsorbed by the slurry. The chlorine addition is terminated when the slurry will no longer react with the chlorine, as evidenced by the presence of a significant amount of chlorine being evolved from the slurry even when it is added to the slurry at a low rate.